1. Field of the Invention
The present invention relates to a coating apparatus and method for applying a liquid to a semiconductor wafer, and more particularly to the mechanism of nozzles for supplying a resist or developing solution for coating onto a semiconductor wafer.
2. Description of the Related Art
As the integration degree of semiconductor devices, such as ICs and LSIs, is increasing, working processes tend to be subdivided. Accordingly, a coating process for applying a resist to a semiconductor wafer and a developing process are complicated.
A spin coater is conventionally used in the resist and developer coating processes. To fulfill requirements of the complicated coating processes, the spin coater is provided with a plurality of liquid supply nozzles. More specifically, one and the same coater is provided with a plurality of resist supply nozzles (for high-resolution resists, dye-mixed resists, resists with high thermal resistance, etc.) and a nozzle for the supply of a developing solution. A required one of the nozzles is selected corresponding to each process by means of a nozzle operating mechanism, and a desired liquid is dripped from the selected nozzle onto a semiconductor wafer.
In a prior art coating apparatus, as shown in FIG. 1, air cylinder 2 and guide 2a are arranged on base 20 so as to extend along the surface of the base. A rod of cylinder 2 is coupled to one end portion of arm 3, while two nozzles 4 and 5 are attached to the other end portion (distal end portion) of the arm.
When nozzles 4 and 5 need not be operated, in this conventional apparatus, arm 3 is moved to one side of base 1, where it is kept on stand-by at a stand-by position, as indicated by two-dot chain line in FIG. 1. When nozzle 4 or 5 is expected to be operated, arm 3 is moved toward the center of base 1 so that nozzle 4 or 5 is situated directly over the center position of semiconductor wafer 7. Wafer 7 is retained by means of a wafer chuck (not shown) which is located inside cup 6. Arm 3 is stopped at a predetermined position, and a predetermined amount of resist is supplied (e.g., dripped or jetted) from, e.g., nozzle 4 onto the center position of wafer 7. Then, semiconductor wafer 7 is rotated to disperse the resist uniformly over the whole surface of the wafer. One of nozzles 4 and 5 is alternatively selected by controlling the stop position of arm 3.
In the coating apparatus described above, however, there is a possibility of the resist unexpectedly falling from the nonoperating nozzle (e.g., nozzle 5) onto the semiconductor wafer. Moreover, the resist may possibly be dried in the liquid passage of the unused nozzle, so that viscosity of the liquid increases too much for even coating. Thus, a resist layer with a desired uniform thickness cannot be formed on the surface of the wafer.
FIG. 2 shows another prior art coating apparatus. In the apparatus of this type, cup 13 having a wafer chuck (not shown) is disposed substantially in the center of base 8, and first and second arms 10 and 15 are arranged on either side of cup 13. One end portion of each arm is supported by means of a pivot, which is coupled to the driving shaft of motor 9 or 14 by means of a gear or a belt. First and second nozzles 11 and 16 are attached to the other end portions (distal end portions) of first and second arms 10 and 15, respectively. Resist reservoirs 12 and 17 are disposed under the distal ends of nozzles 11 and 16, respectively, whereby the resist in each nozzle is prevented from being dried.
In this conventional coating apparatus, one of nozzles 11 and 16 is alternatively selected. If first nozzle 11 is selected for use, for example, arm 10 is rocked around its pivot so that nozzle 11 is situated directly over the center position of semiconductor wafer 7. Then, a predetermined amount of resist is dripped from nozzle 11 onto wafer 7, and the wafer is rotated to disperse the resist uniformly over the whole surface of the wafer.
In the coating apparatus of this type, however, the nozzle operating mechanism is increased size and complicated in construction as the number of nozzles increase. If the nozzles increase in number, moreover, the arms interfere with each other, so that reliable operation cannot be ensured.
The liquid passage of each liquid supply nozzle has a special shape such that air bubbles are prevented from being produced in the liquid, in order to keep the amount of liquid dripped onto the semiconductor wafer constant. Since the liquid temperature also influences the thickness of a coating film, the liquid in the nozzles sometimes may be adjusted to a suitable temperature.
The conventional nozzle contains therein a tube made of fluoroethylene resin, e.g., PFA (tetrafluoroethylene resin) or PTFE (soft tetrafluoroethylene resin), and the distal end portion of the resin tube is used as a liquid supply passage.
In the conventional nozzle, however, the resist adheres to a thick-walled portion of the distal end portion of the tube, and it dries and solidifies, thus narrowing a liquid supply port, so that the amount of dripped liquid varies.
While handling the semiconductor wafer or in replacing the cup of the coating apparatus, furthermore, the nozzle may run against the wafer, cup, or other members, to be deformed or broken thereby. If the nozzle is damaged in this manner, its replacement, mounting, and adjustment require a lot of time, since the nozzle is a tube which constitutes part of pipe arrangement.
In order to solve these problems of the conventional nozzle, an improved nozzle has been developed in which nozzle tip 11 for liquid discharge is attached to the distal end portion of stainless-steel tube 10, as shown in FIG. 3, so that a liquid is dripped through tip 11. Since the distal end portion of tip 11 is thin-walled, the liquid can never adhere to the tip end. Nozzle tip 11 may be formed of a tube of fluoroethylene resin, such as tetrafluoroethylene resin (PFA) or trifluorochlorethylene resin (PCTFE), for example.
In the prior art nozzle described above, however, tube 10 is made of stainless steel, so that air bubbles in the tube cannot be visually observed. Since nozzle tip 11 is attached to the distal end of tube 10 by press fit, moreover, the press-fit portion will be deteriorated after prolonged use. Air may penetrate the liquid passage through the loosened press-fit portion, thus producing air bubbles.